In recent years, various optical media have been developed, and an optical pickup device that can record/replay using multiple types of common optical recording media is known. For example, a device that records/replays using one optical pickup device with either a DVD (Digital Versatile Disc) recording medium or a CD (Compact Disc including CD-ROM, CD-R and CD-RW) recording medium is known. In order to improve the recording density using a DVD, it is required to use visible light having, for example, a wavelength of approximately 650 nm. On the other hand, because a CD does not have any sensitivity for light in the visible light region, it is necessary to use near-infrared light having a wavelength of approximately 780 nm. Therefore, optical pickup devices that can be used with either of these media rely on a so-called two-wavelength beam method wherein two different wavelengths are used to irradiate the recording medium for recording/playback.
In addition, in the two optical recording media devices as discussed above, it is necessary to employ different numerical apertures depending on the wavelength used. For example, in the technical standards for DVDs, the numerical aperture is established at 0.6, while in the technical standards for CDs, the numerical aperture is established at 0.45. Consequently, in order to obtain the required different numerical apertures for the selected optical recording media, either a variable aperture diaphragm is interposed that may be formed of a liquid crystal shutter or a filter having a wavelength selectivity, or different diaphragms are mechanically switched into the light path of the irradiating beam. The inventors of the present application have already disclosed a construction in Japanese Laid-Open Patent Application 2002-117569 wherein a peripheral portion, which functions to reduce/eliminate light at the periphery of a central light flux due to interference for light having one wavelength and also functions to maintain a quantity of light at the periphery of a central light flux for light having the other wavelength, is formed on one surface of an objective lens. Using such an arrangement obviates the need for a diaphragm as used in the prior art to provide different numerical apertures, so that a compact size and a low price for an optical pickup device is realized.
In the two optical recording media mentioned above, the thickness of the optical disc is 0.6 mm in the case of using a DVD versus 1.2 mm in the case of using a CD. As mentioned previously, because the thicknesses of the discs in the two optical recording media are different, the thickness of the protective layers are different, and thus the amount of spherical aberration generated will be different for DVD versus CD recording/replaying.
In the objective lens and the pickup device of Japanese Laid-Open Patent Application 2002-117569, the construction in the case of recording/replaying using a DVD is such that a substantially collimated laser beam is incident onto the objective lens. On the other hand, in the case of recording/replaying using a CD, the construction is such that a slightly diverging laser beam is incident onto the objective lens. Consequently, even though the amount of spherical aberration that is generated varies when using a DVD versus a CD as the recording/replaying medium, the construction is such that the laser beam can be converged to a proper spot size for recording/replaying using a laser beam of an appropriate wavelength for the selected medium while using the same objective lens.
If a substantially collimated light beam is incident onto the objective lens when using a DVD, the construction is such that an optimum converging effect is provided by the objective lens. At the same time, a light source corresponding to the other optical recording medium (for example, a CD) is arranged at a position so as to provide the best optical performance possible for CDs. Therefore, the position of the light source corresponding to the other optical recording medium is limited. This results in the degree of freedom in designing the optical pickup being low and also requires accuracy in assembly. However, if a substantially collimated light beam were to be incident onto both optical recording mediums, and if the objective lens could display the optimum converging effect for each optical recording medium, the degree of freedom in designing the optical pickup would become high. Such a design would also be effective for achieving a compact size, for easing the accuracy of assembly that is required, and for lowering costs.
The present invention relates to an objective lens for an optical recording medium that can efficiently converge either of the two different irradiating beams required when using a device that can record onto either a DVD or CD wherein the numerical aperture of the used optical system and the wavelength of the radiation used differ from each other. Also, the present invention relates to an optical pickup device that uses such an objective lens.